Mitchell order
In mathematical set theory, the Mitchell order is a well-founded preorder on the set of normal measures on a measurable cardinal κ. It is named for William Mitchell. We say that M ◅ N (this is a strict order) if M is in the ultrapower model defined by N. Intuitively, this means that M is a weaker measure than N (note, for example, that κ will still be measurable in the ultrapower for N, since M is a measure on it).
In fact, the Mitchell order can be defined on the set (or proper class, as the case may be) of extenders for κ; but if it is so defined it may fail to be transitive, or even well-founded, provided κ has sufficiently strong large cardinal properties. Well-foundedness fails specifically for rank-into-rank extenders; but Itay Neeman showed in 2004 that it holds for all weaker types of extender.
The Mitchell rank of a measure is the ordertype of its predecessors under ◅; since ◅ is well-founded this is always an ordinal.
A cardinal which has measures of Mitchell rank α for each α < β is said to be β-measurable.
References
- John Steel (Sep 1993). "The Well-Foundedness of the Mitchell Order". Journal of Symbolic Logic. 58 (3): 931–940. doi:10.2307/2275105.
- Itay Neeman (2004). "The Mitchell order below rank-to-rank". J. Symbolic Logic. 69 (4): 1143–1162. doi:10.2178/jsl/1102022215.
- Akihiro Kanamori (1997). The higher infinite. Perspectives in Mathematical Logic. Springer.
- Donald A. Martin and John Steel (1994). "Iteration trees". Journal of the Americal Mathematical Society. 7: 1–73.
- William Mitchell (1974). "Sets constructible from sequences of ultrafilters". Journal of Symbolic Logic. 39: 57–66. doi:10.2307/2272343.